The present invention relates to optical testing apparatus. More specifically the invention relates to apparatus for determining the refractive power(s) of lenses, including spherical power and cylindrical power.
During or after their manufacture, lenses, and particularly ophthalmic lenses, are conventionally subjected to various tests to insure that they possess the optical characteristics for which they were designed. For example, ophthalmic lenses are generally tested to at least determine their refractive powers. For a lens having only spherical power, the refractive power will be the same in all meridians; however, a lens may additionally include a cylindrical power component, either intentionally or unintentionally, and thus exhibits one refractive power in one principal meridian of the lens and a different refractive power in the other principal meridian orthogonal thereto.
The measurement of refractive power(s) of ophthalmic lenses has typically been performed with a lens measuring instrument in which an operator positions the test lens and, looking at the eyepiece, rotates and translates a target until one set of lines of the target is in good visual focus. A dial on the instrument then indicates the power in diopters. If the lens is toric and has a cyclindrical component, the operator continues to translate the target until another set of lines thereof (orthogonal to the first set) is visually in focus. The power of that meridian is then read. In such a device, the power of the lens is a linear function of the position of the movable target.
It will be appreciated that the use of this type of lens measuring instrument is slow and is subject to the inaccuracies introduced by the determination of a "focussed" condition using the human eye and mind. Further, the operation of such instrument requires a trained experienced operator.
In U.S. Pat. No. 3,323,417 to Grey et al. a technique is described in which a light beam is passed through a reticle or object pattern, such as a grid, and the lens to be tested is used as a part of an optical system which focuses and reflects the image of the grid back through the grid to a photo responsive cell. Electrical signals are generated in response to the amount of light reflected back through the grid, and those signals are at a maximum when the reflection of the grid image is focused on the grid. Displacement of the test lens from a reference position to a position at which the grid image is focused on itself is indicative of the focal length of the lens. The principal of retro-reflection serves to cancel the effects of possible prism in the system.
The technique of the aforementioned patent is discussed in connection with lenses typically for photographic applications and normally possessing only sphere power. The "suitable object pattern" is specifically described as being a "grid system having a plurality of slits." While such an arrangement may be satisfactory for the determination of power in a lens having only sphere power, it is less than satisfactory if the lens also possesses cylindrical power, as would be the case for correction of astigmatism. Assuming slits extending in only one direction, the pattern will be clearly imaged on itself only for the principal meridian of the lens extending parallel to the slit, thereby requiring 90.degree. rotation of the lens or target if both meridians are to be separately clearly imaged. Even if the object pattern is provided with orthogonally extending slits, it is necessary to place the principal meridians of the lens in alignment with the object pattern.
Other systems which are capable of objective and accurate determination of spherical and cylindrical lens power are handicapped by their expense and/or complexity and/or limitation of the lens power measurement range.